The present invention relates to electric motors and in particular, to an internal cooling fan for a permanent magnet direct current (PMDC) electric motor.
There is a constant demand to increase the power output of small motors for a given motor size. When such small motors are required to deliver large output powers, there is a difficulty in dissipating the heat generated inside the motor through power losses. These motors have small masses and so, can experience very rapid rises in temperature in critical areas. At excessive temperatures, various components are subject to damage.
To keep these motors below critical temperatures, ventilation fans are mounted to the rotor, either on the shaft, on the commutator or directly to the rotor core. Generally, the fans are of a centrifugal type with straight, radial fan blades to cool the motor when the motor is operating in either clockwise or anti-clockwise rotation.
A particular fan for a PMDC motor is shown in GB 2385715. This fan design was developed to address a concern that the air flow generated by the average fan drew too much air from over the commutator and insufficient air through the rotor core and over the rotor windings due to significant differences in the resistance of the air flow paths. However, that fan design was impractical to manufacture for use in a low cost, high production motor and so the need still remains for a fan which is conveniently mounted to the rotor after assembly of the rotor and which can draw a significant volume of air through the rotor core and over the rotor windings while still drawing air over the commutator.